DESCRIPTION: Since thrombocytopenia is a serious life threatening complication of cancer and AIDS chemotherapy, standardized and optimized human assays for human megakaryocyte progenitors would be useful for determining and/or predicting the toxicity of experimental chemotherapeutic drugs. The proposed experiments in this Phase II application use megakaryocyte and stem cell populations isolated by FACS and analyzed by flowcytometry to evaluate the toxicity of anticancer and anti HIV drugs on megakaryocytes and their progenitors. The mechanism of drug toxicity will be explored using these cells to distinguish whether the toxic effects of the drugs are due to a direct effect on megakaryocytes or on more primitive hematopoietic precursor cells. The in vitro toxicity data will be correlated with in vivo toxicity data from patients who receive the same drugs.This will enable a determination of the predictive accuracy of the in vitro data in the same way that drug toxicity to CFU GM and CFU E/BFU E has been shown to correlate with in vivo myelotoxicity using assays developed at Hipple Cancer Research Center. In addition, the Investigators propose to test for drug toxicity using long term bone marrow cultures as an in vitro model of steady stay hematopoiesis. This system is proposed to allow the capability of identifying the mechanisms underlying early versus late onset thrombocytopenia. In addition, the proposed research may provide methods for optimal growth of primitive hematopoietic cells and for selecting the optimal drug combination for mobilizing bone marrow stem cells to the peripheral blood for clinical transplantation, predictions which can be tested clinically.